Uncertainties in isotopic ratio measurements of individual silver nanoparticles using multiple-collector ICP-mass spectrometry

Abstract

A critical evaluation of measurement uncertainties for the isotopic ratio of individual silver nanoparticles (Ag NPs) was performed using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) equipped with high-gain Faraday detectors with 1013-ohm amplifiers. In this study, a continuous ion monitoring (CIM) protocol was used to minimise the influence of the slow response of the Faraday amplifiers. The measured 109Ag/107Ag ratios were 0.938 ± 0.209 for 40 nm Ag NPs, 0.948 ± 0.141 for 60 nm Ag NPs, 0.943 ± 0.051 for 80 nm Ag NPs, 0.934 ± 0.029 for 100 nm Ag NPs, and 0.938 ± 0.011 for 200 nm Ag NPs, corresponding to relative deviations from the natural isotopic ratio of 0.54% (40 nm), 2.04% (60 nm), 1.51% (80 nm), 0.54% (100 nm), and 0.97% (200 nm), respectively. The expanded relative uncertainty (k = 2) for the 109Ag/107Ag ratio of individual Ag NPs ranged from 15.3% for 40 nm Ag NPs (i.e., ca. 10,000 counts/particle event) to 0.66% for 100 nm Ag NPs (i.e., ca. 600,000 counts/particle event), primarily dictated by the signal intensities of the Ag isotopes. The major sources of uncertainty were (a) counting statistics of signals generated by individual NPs, (b) background ion currents, and (c) fluctuations in background ion currents. The results demonstrate that increasing signal intensity and minimising background signals are critical for reducing uncertainty and achieving high-precision isotopic ratio measurements of individual NPs